My biggest issue is that any class can do EVERYTHING.
Mr_Gyactus referred THM as being able to Heal, DPS, Debuff, however THM can do so much more than just this because there are not heavy penalty restrictions from cross classing abilities and giving titles, jobs would enhance this greatly and give more customization than we already have so arguing that they like having a sandbox isn't valid because it would still exist but WITH MORE OPTIONS ADDED TO IT.
For example I can do the exact same thing as CON magic wise if I equip the exact same actions. So what is the difference in me using THM or CON? The only difference is which AOE style I prefer and that's not a bid deal at all and the only reason I would even consider leveling both or 1 over the other is which actions I prefer which if I level both which I use comes down to which AOE I prefer (NOT MUCH CUSTOMIZATION NOW IS THERE?).
Now also I can equip melee DPS skills on THM as well such as Doomspike from LNC. Now my highest damage output with Doomspike as LNC while using 2 attack buffs and 2 accuracy buffs at 3000 TP is 600 damage solo. Now as THM with the exact same actions equipped, same gear, same attributes my highest damage using Doomspike is also 600 damage with 3000 TP. So the exact same damage and I can attack form range with Doomspike as THM (DOES THIS LOOK LIKE CUSTOMIZATION TO YOU BY ONLY DIFFERENTIATING WHICH WEAPON I PREFER?).
Now lastly we can move on to THM tank role. Let's compare to GLA since people see this as the tank role. Well both get a shield, can use the same actions shield or not. Have the exact same effectiveness on all actions including all enmity, damage mitigation, and self-curing effectiveness. The only differences between these two are which weapon do I prefer and do I prefer twice as much HP or being able to AOE spells. Now there is a much bigger difference here than my two previous examples ONLY because of the HP difference which makes tanking essentially twice as difficult however is just as effective because they take a very similar amount of damage with the same actions equipped and being able to cast cure just slightly faster makes up for taking just slightly more damage. The oonly thing that would prevent THM from tanking over a GLA is if the THM gets "1 shot" which is highly more likely because of the HP difference and that is it. Now let's compare being able to AOE spells which GLA can't do. As THM I can AOE Paralyze/Sleep/Absorb ATK/ Absord DEF in a behest and aggro every single mob and them spam cures on myself and tank every single mob. I can also use Wardrum as THM since I can equip a shield. Now if I have just as many people curing me as THM as people would be curing a GLA tanking this many mobs at once time then I will be fine especially after having ATK and DEF weakened on ALL mobs and mine increased as where as GLA could only do this on 1 mob (SO IS THERE STILL A HUGE DIFFERENCE IN CUSTOMIZATION BETWEEN WHICH WEAPON YOU USE AND THE OPTION OF HP OR AOEING SPELLS?).
This is my biggest issue with the game ATM because I can do everything as every class in the same way with the same effectiveness.
Edit: Another thought is that maybe the Title System can be worked to allow every class to perform every role just not the exact same way so there is a reason to use one class over the other based on player preference.